As known, a flush valve is such a flow channel opening/closing apparatus. The flush valve comprises a main body section having a flow inlet for receiving water from a primary-side flow channel, which is a water supply source, and feeding the water to a primary-side internal flow channel and a flow outlet for feeding water from a secondary-side internal flow channel to a secondary-side flow channel, which is a water supply destination, a main valve (a diaphragm valve) for opening and closing a flow channel between the primary-side internal flow channel and the secondary-side internal flow channel, a bypass flow channel that connects the primary-side internal flow channel and the secondary-side internal flow channel to each other without via the main valve, and a sub-valve (a relief valve) for opening and closing the bypass flow channel (see Patent Literature 1 listed below, for example).
With the flush valve configured as described above, when the sub-valve is opened by pushing down the operating lever, for example, the bypass flow channel is opened to decrease the back pressure on the main valve body of the main valve, the primary pressure in the primary-side internal flow channel pushes up the main valve body to separate the main valve body from the main valve seat, thereby opening the main valve, and water flows out to the secondary-side flow channel through the flow outlet. Then, when the sub-valve is closed by putting the operating lever back to the original position or automatically returns to the original position, the bypass flow channel is closed to increase the back pressure on the main valve body. The increased back pressure causes the main valve body to move downward and come closer to the main valve seat until the main valve body comes into contact with the main valve seat to close the main valve. In this way, the flush valve serves as a flow channel opening/closing apparatus that starts water supply to the toilet bowl in response to receiving an instruction to start water supply and autonomously stops water supply when a predetermined condition is met.
Conventional flush valves are quite useful apparatuses that feed approximately a predetermined amount of water with relatively simple structures and are widely used as means of supplying water to a urinal or toilet bowl. However, the conventional flush valves can hardly strictly control the amount of water because of their structures. Japanese Industrial Standards (JIS) prescribe that the normal water discharge amount is 15 L, the acceptable water discharge amount at low water pressure is 11 to 16.5 L, and the acceptable water discharge amount at high water pressure is 13.5 to 19 L.
As described above, with the conventional flush valves, the water discharge amount varies depending on the water pressure. In public spaces, for example, a plurality of toilet bowls is typically installed side by side, so that the water pressure can remarkably vary depending on the utilization of the toilet bowls. To overcome the problem, conventional flush valve type toilet bowls are configured to supply more water than required to ensure that feces are washed away even when the water pressure is low or the water pressure significantly varies. Thus, particularly in an environment where the water pressure is high or the water pressure less significantly varies, an unnecessarily increased amount of water has to be uselessly fed, and there was a demand for a solution to save water.
To solve the problem, there has been proposed a technique of incorporating a constant flow rate valve into the flush valve to keep the flow rate of water fed to the secondary-side flow channel constant even in an environment where the water pressure is high or even when the water pressure significantly varies in the primary-side flow channel, thereby reducing the waste of water and improving the water saving capability (see Patent Literature 2 listed below).